The survival and maintenance of differentiated function of vertebrate neurons is influenced by the availability of specific proteins referred to as neurotrophins. Developing neurons depend for survival on the supply of these factors from their target fields and the limited production of neurotrophins results in death of superfluous neurons (for reviews, see (1); (2)). The various neurotrophins differ functionally in their ability to support survival of distinct neuronal populations in the central and the peripheral nerve system (3), (4); (5), (80).
The neurotrophin family is a highly homologous family which includes NT3 (6), (7); (5); (8); (9); (10), nerve growth factor (NGF) (11); (12), brain-derived neurotrophic factor (BDNF) (13); (14)) and neurotrophin 4/5 (NT4/5) ((15), (16), (17).
Studies suggest that neurotrophins transduce intracellular signalling at least in part through the ligand-dependent activation of a class of tyrosine kinase-containing receptors of Mr=140–145,000 known as the trks (18); (19) (21); (20) (22); (23); (24); (25); (26). Thus, the signal transduction pathway of neurotrophins is initiated by this high-affinity binding to and activation of specific tyrosine kinase receptors and subsequent receptor autophosphorylation (19); (27). Although there is some degree of cross-receptor interaction between the neurotrophins and the different trks, the predominant specificity appears to be NGF/trkA, BDNF/trkB, and NT3/trkC while NT4/5 appears to interact primarily with trkB as efficiently as BDNF (27); (19) (21); (25); (22); (28); (18); (28a). While trkC responds exclusively to NT3 (25); (26), trkA and trkB can respond in vitro under certain circumstances to multiple neurotrophins (6); (23). However, the neuronal environment does restrict trkA and trkB in their ability to respond to non-preferred neurotrophic ligands (29). In addition to the trk family of receptors, the neurotrophins can also bind to a different class of receptor termed the p75 low affinity NGF receptor (p75; (30); (31)) which has an unknown mechanism of transmembrane signalling but is structurally related to a receptor gene family which includes the tumor necrosis factor receptor (TNFR), CD40, 0×40, and CD27 (32); (33); (34), (35); (36); (37)). The role of the gp75 in the formation of high-affinity binding sites and in the signal transduction pathway of neurotrophins is as yet unclear (for reviews see (38); (39)).
An examination of the primary amino acid sequence of the neurotrophins reveals seven regions of 7–10 residues each which account for 85% of the sequence divergence among the family members.
Nerve growth factor (NGF) is a 120 amino acid polypeptide homodimeric protein that has prominent effects on developing sensory and sympathetic neurons of the peripheral nervous system. NGF acts via specific cell surface receptors on responsive neurons to support neuronal survival, promote neurite outgrowth, and enhance neurochemical differentiation. NGF actions are accompanied by alterations in neuronal membranes (40), (41), in the state of phosphorylation of neuronal proteins (42), (43), and in the abundance of certain mRNAs and proteins likely to play a role in neuronal differentiation and function (see, for example (44)).
Forebrain cholinergic neurons also respond to NGF and may require NGF for trophic support. (45). Indeed, the distribution and ontogenesis of NGF and its receptor in the central nervous system (CNS) suggest that NGF acts as target-derived neurotrophic factor for basal forebrain cholinergic neurons (46), (81).
Little is known about the NGF amino acid residues necessary for the interaction with the trkA-tyrosine kinase receptor. Significant losses of biological activity and receptor binding were observed with purified homodimers of human and mouse NGF, representing homogenous truncated forms modified at the amino and carboxy termini. (47); (48); (49). The 109 amino acid species (10–118)hNGF, resulting from the loss of the first 9 residues of the N-terminus and the last two residues from the C-terminus of purified recombinant human NGF, is 300-fold less efficient in displacing mouse [125]NGF from the human trkA receptor compared to (1–118)HNGF (49). It is 50- to 100-fold less active in dorsal root ganglion and sympathetic ganglion survival compared to (1–118)hNGF (48). The (1–118)HNGF has considerably lower trkA tyrosine kinase autophosphorylation activity (49).
NT3 transcription has been detected in a wide array of peripheral tissues (e.g. kidney, liver, skin) as well as in the central nerve system (e.g. cerebellum, hippocampus) (5); (7), (82). During development, NT3 mRNA transcription is most prominent in regions of the central nervous system in which proliferation, migration and differentiation of neurons are ongoing (50). Supporting evidence for a role in neuronal development includes the promoting effect of NT3 on neural crest cells (51) and the stimulation of the proliferation of oligodendrocyte precursor cells in vivo (79). NT3 also supports in vitro the survival of sensory neurons from the nodose ganglion (NG) (7); (5), (83) and a population of muscle sensory neurons from dorsal root ganglion (DRG) (52). In addition to these in vitro studies, a recent report showed that NT3 prevents in vivo the degeneration of adult central noradrenergic neurons of the locus coerulus in a model that resembles the pattern of cell loss found in Alzheimer's disease. Currently, there are no published reports concerning the amino acid residues necessary for trkC binding.
There has been some limited attempts to create chimeric or pan-neurotrophic factors. (See (53); (56); (54), (55)).